The present invention relates to a power supply apparatus for use in electrostatic recording apparatuses such as copiers and printers.
An example of a power supply apparatus of a conventional art is shown in FIG. 5.
In FIG. 5, transformer 1 has primary winding 1A and secondary winding 1B. One end of primary winding 1A is connected with first DC power source 2 and the other end of primary winding 1A is connected with transistor 3 as a switching element. Diode 4 is connected in parallel with the collector and the emitter of transistor 3. The anode of diode 4 is grounded.
Voltage doubler rectifier circuit 5 is formed of rectifier diodes 5A, 5B, smoothing capacitors 5C, 5D, and discharging resistor 5E. The input terminal of voltage doubler rectifier circuit 5 is connected with secondary winding 1B of transformer 1. One of the outputs of voltage doubler rectifier circuit 5 is connected with output terminal 6 of the power supply apparatus and the other of the outputs of voltage doubler rectifier circuit 5 is grounded through current detector 7.
Current detector 7 is connected with one of the comparison inputs of error amplifier 9 and the other of the comparison inputs of error amplifier 9 is connected with second DC power source 10. The output of error amplifier 9 is fed into control circuit 8 and the output of control circuit 8 is connected to the base of transistor 3. Control circuit 8, in receipt of the output signal of error amplifier 9, can vary a signal fed to the base of transistor 3.
Then, voltage detector 11 is connected to the junction point between primary winding 1A and transistor 3. The output of voltage detector 11 is connected to control circuit 8 through limiter 12. Limiter 12, when an input thereto exceeds a predetermined operating level, outputs a signal for limiting the output of control circuit 8.
Operation will be described below. A DC voltage supplied from first DC power source 2 to primary winding 1A of transformer 1 is subjected to switching in transistor 3 and thereby an AC voltage is induced in secondary winding 1B. The generated AC voltage is doubled and rectified in voltage doubler rectifier circuit 5, whereby a DC voltage whose voltage value is the same as the amplitude voltage of the AC voltage generated in secondary winding 1B is generated at output terminal 6 of the power supply apparatus.
When a load is connected to the power supply apparatus and a current is allowed to pass therethrough, a current of the same value as the load current is fed through current detector 7 to have a voltage generated therein. The generated voltage is fed into the comparison input of error amplifier 9 and compared with the voltage of third DC power source 10.
Based on a result of the comparison, a signal to the base of transistor 3 is varied through control circuit 8 and control is performed such that the voltage of third DC power source 10 and the voltage generated in current detector 7 become the same in value.
Thus, the load current of the power supply apparatus is kept constant.
Then, when the impedance of the load becomes higher, the voltage at output terminal 6 becomes higher and the voltage in primary winding 1A of the transformer is elevated. If, at this time, the voltage of voltage detector 11 exceeds a limited operating level of limiter 12, limiter 12 supplies a signal to control circuit 8 to limit the output, whereby abnormal rise of the output voltage of the power supply apparatus is limited and a dielectric breakdown is prevented.
In the above described configuration, the voltage waveform in secondary winding 1B comes to have ringing portions at both the forward period and the flyback period as shown in FIG. 4C. However, the voltage waveform in primary winding 1A, though it is analogous to the waveform in secondary winding 1B at the flyback period, is brought to grounded potential at the forward period, as shown in FIG. 4A, because the ringing portion is clipped by diode 4 at the forward period.
The effect of the difference between the voltage waveform in secondary winding 1B and the voltage waveform in primary winding 1A on the load limiting characteristic will be described below.
When the load current is decreased with increase in the load impedance, since the voltage waveform in secondary winding 1B is such that has ringing portions both at the forward period and at the flyback period, similarly to the waveform shown in FIG. 4C, the output voltage of voltage doubler rectifier circuit 5 becomes the sum of the voltage peak value at the forward period and the voltage peak value at the flyback period.
However, the voltage waveform in primary winding 1A has the voltage peak value only at the flyback period as shown in FIG. 4A because the ringing portion at the forward period is clipped by diode 4. Therefore, the voltage becomes lower than the output voltage which is inherently expected. Accordingly, the voltage on which limiter 12 operates becomes different from the output voltage on which it is inherently expected to operate.
Consequently, inclination is produced in the load limiting characteristic as indicated by A in FIG. 3 and hence the output voltage is increased at the time when the load current decreases and of no load. As a result, it is required to provide excessive insulation and this has been a problem in producing power supply apparatuses and peripheral equipment thereof smaller in size.
There is provided a power supply apparatus which comprises: a transformer having a primary winding and a secondary winding; a first DC power source connected to one terminal of the primary winding of the transformer; a diode whose anode is connected to the other terminal of the primary winding of the transformer; a switching element connected between the cathode of the diode and the ground; a voltage doubler rectifier circuit connected to the secondary winding of the transformer; a current detector for detecting a DC current from the voltage doubler rectifier circuit; a control circuit for controlling the switching element in accordance with an output from the current detector; an AC voltage detector connected in parallel between a junction point between the anode of the diode and the other terminal of the primary winding of the transformer and the ground; and a limiter for controlling the switching element when the output of the AC voltage detector exceeds a predetermined voltage value thereby limiting voltage of the secondary winding of the transformer.